One of the best tests of character as a scientist is how you respond when a colleague asks you a pointed question that you don’t have a great response for. This is especially true when different colleagues ask you the same question. For me, that inquiry has been, “Why doesn’t your mouse model of urogenital schistosomiasis result in bladder cancer?”

Before I delve into why this observant question is painful for me, I should rewind and paint the backdrop associated with this recurring query. Urogenital schistosomiasis, infection by parasitic Schistosoma haematobium worms, affects approximately 112 million people worldwide. Most of the burden of disease is in sub-Saharan Africa, with some distribution in the Middle East, and possibly even parts of Mediterranean Europe. Urogenital schistosomiasis causes an incredible range of human disease, but perhaps its most notorious manifestation is bladder cancer.

Although urogenital schistosomiasis is an accepted risk factor for bladder cancer, and in fact may be one of the most important risk factors besides smoking, the biological pathways linking S. haematobium infection to bladder carcinogenesis are poorly understood. This is in no small part due to the fact that traditionally, animal models for urogenital schistosomiasis have been lacking. Hamsters can be infected with S. haematobium, but they almost always develop liver and intestinal disease, rather than bladder infection like that seen in humans. Non-human primates are biologically compatible hosts for S. haematobium, but they are very expensive and their use is coming under increasing scrutiny due to ethical considerations. There are also very few scientific tools specifically geared towards use with hamsters and non-human primates, which further exacerbates the problem. In contrast, although there is a large array of scientific tools to study mouse biology, mice, like hamsters, develop liver and intestinal disease after S. haematobium infection.

To attempt to circumvent these barriers to urogenital schistosomiasis research we developed techniques to inject S. haematobium eggs, the parasite life stage which triggers human bladder changes, into the bladder walls of mice (Fu et al.). This approach recapitulates many of the changes seen in the human bladder infected by S. haematobium (reviewed by Payne and Hsieh), including urothelial hyperplasia and squamous metaplasia of the inner lining of the bladder:

The squamous metaplasia seen in our mouse model is relevant, given that similar findings are observed in many human bladders with squamous cell carcinoma, an otherwise unusual form of bladder cancer that is highly prevalent in schistosomiasis-endemic regions:

Schistosoma haematobium-associated bladder squamous metaplasia and carcinoma. Micrographs from stained sections of a bladder with keratinized, moderately differentiated squamous cell carcinoma associated with urogenital schistosomiasis. (A) Low power view of bladder section. (B) High power view of area indicated by broken line box in (A) demonstrating squamous metaplasia of the urothelium with infiltration of the lamina propria by a large number of S. haematobium ova (several eggs are circled as examples). In this specimen, the squamous metaplasia is evident as a hyperkeratotic squamous epithelium (arrow) lining the bladder lumen. (C) Another region of the same bladder exhibits abundant keratin pearls (examples indicated by arrows), a classic sign of squamous cell carcinoma. Adapted from Honeycutt et al.

Importantly, our mouse model indeed does not result in frank bladder carcinogenesis, despite the development of potentially pre-cancerous changes such as squamous metaplasia and urothelial hyperplasia. There are a number of plausible reasons why this is the case. First, our mouse model features a single injection of eggs into the bladder wall. Egg deposition in infected humans occurs continuously over the course of years to decades. By definition, our approach cannot reproduce the kinetics of human infection. Since cancer is a very chronic disease that develops slowly, and the life span of mice is measured in a few years at best, it is also likely that even with continuous egg deposition, mice may not live long enough to develop bladder cancer. Finally, we may not be seeing bladder cancers developing in our mouse model because cancer in general requires “multiple hits”. It is rare for cancer to develop in a given person unless they have, for example, both genetic susceptibility and exposure to carcinogens (i.e., smoking, excessive red meat intake, etc.). In short, people’s bodies need to be “hit” again and again by genetic mutations that accumulate and eventually result in cancer. By analogy, it may be necessary for people with urogenital schistosomiasis to have additional environmental exposures and/or genetic predisposition in order for them to develop bladder cancer. Otherwise, there would literally be tens of millions of people in sub-Saharan Africa with schistosomal bladder cancer.

When asked about why my mouse model doesn’t result in bladder cancer, I don’t go through all the points I’ve listed above. I tailor the answer to the interest level and scientific background of the questioner, but ultimately the deeper question is not whether multiple hits are required for schistosomal bladder cancer to ensue, but which ones and when. If I can contribute just a little bit to understanding how this occurs, that’s a hit that I’ll take.